Recently, there has been a great deal of concern over the problem of air pollution. This problem is particularly acute in the urban areas of the country. There are many sources of air pollution such as the internal combustion engine, chemical processing plants, power generating facilities, etc. One of the more serious pollutants is the oxides of nitrogen, such as NO and NO.sub.2, which are collectively known as NO.sub.x and which contribute to air pollution by the formation of smog. Other pollutants, principally carbon monoxide (CO) and to a lesser extent unburned hydrocarbons (HC), also contribute to the environmental burden. Of all the pollutants resulting from fossil fuel combustion, experience has shown that NO.sub.x is one of the most difficult to minimize. Even though reductions in NO.sub.x are difficult to achieve, recently enacted pollution abatement standards as set forth in Rule 1146.1 of the South Coast Air Quality Management District (SCAQMD) establishes emission limits of 30 parts per million by volume (ppmv) for NO.sub.x and 400 ppmv for CO, both levels corrected to 3% exhaust oxygen content.
In fuel burning facilities, such as power generating stations, there are various sources of NO.sub.x emissions. One source of NO.sub.x emissions, referred to as thermal NO, results from the oxidation of the diatomic nitrogen (N.sub.2) component in the combustion air. Thermochemistry requires temperatures typically in the order of 2800.degree. F. (1810.degree. K.) for the formation of NO in this manner. The diatomic nitrogen (N.sub.2) component must first be dissociated into atomic nitrogen (N) prior to the formation of NO. Another source of NO.sub.x emissions, referred to as fuel NO, results from the fact that many fuels contain the single atomic nitrogen species, for example, ammonia (NH.sub.3). In this case, N.sub.2 bond splitting is not a prerequisite for NO formation thereby allowing conversion of fuel-bound nitrogen into NO at temperatures significantly below 2800.degree. F. (1810.degree. K.). Conversion of fuel-bound nitrogen into NO can occur at temperatures as low as 1300.degree. F. (977.degree. K.). Still another source of NO.sub.x emissions, referred to as prompt NO, results from high-speed reactions. Formation of NO by high speed reactions in fuel rich zones in the flame front have been reported and is the subject of ongoing research. No widely accepted mechanism for this phenomena has been developed.
In those geographic areas where stringent air quality control regulations have been enacted, such as those areas included within the SCAQMD, it has become extremely difficult to reach the standards established for NO.sub.x emissions by utilizing presently available burners and/or methods of operating same. Various approaches have been developed for reducing NO.sub.x emissions, however, the resulting reduction in emissions is not sufficient in many cases to satisfy the foregoing stringent air quality standards. Some of these approaches are based on reducing NO.sub.x emissions by multi-stage combustion. For example, such multi-stage combustion might involve burning a first fuel as a "lean mixture" and subsequently burning the resulting combustion products with a second fuel to form an atmosphere which causes a reduction in NO.sub.x emissions. Alternatively, fuel and air can be introduced into a burner so as to form two separate streams each having different of fuel to air ratios, i.e., one stream would have an excess of air while the other stream would have an excess of fuel. One of the streams is then ignited effecting a first stage of combustion which then ignites the second stream effecting a second stage of combustion. A third stage of combustion is provided by mixing and burning the excess fuel in one of the streams with the excess air in the other of the streams. A still another approach to reduce NO.sub.x emissions requires a plurality of burners disposed in a series connection with respect to the direction of flow of combustion air. In this case, the last burner in the series of burners utilizes a fuel having lower NO.sub.x producing properties.
Decreasing the temperature of combustion can also result in a reduction in NO.sub.x emissions. The combustion temperature can be reduced by direct flame cooling through water injection of the combustion gases or by adding a cooling gas to the air/gas mixture. Flame temperature can also be reduced by utilizing radiant burners which are, most often, essentially surface combustors employing ceramic fibers, metallic fibers or reticulated ceramic foams as the radiant surface. A major disadvantage of most surface combustors is that because of their large size, a substantial volume of air/gas mixture is trapped within the burner. In the event of flashback, which is a distinct possibility and which adversely affects the applicability of such combustors, the deflagration created may be of explosive proportions. Another disadvantage of surface combustors is that to achieve optimal radiant output for a given input (radiant efficiency), the surface temperature must remain extremely high. Surface combustion temperatures are very sensitive to air/fuel ratio, velocity, and flow uniformity. A reduction in surface temperature diminishes the radiant output by the fourth power which would likely result in higher NO.sub.x emissions levels, via higher flame temperatures.
NO.sub.x emissions can also be reduced by recirculating the flue gases within the combustion chamber. In this approach, a portion of the flue gases can either be mixed with the combustion air prior to combustion, or delivered into the combustion zone separately. The recirculated flue gas acts as a diluent to lower the overall oxygen concentration and flame temperature. In essence, the combustion air supply is vitiated, thus reducing NO.sub.x, however, carbon monoxide (CO) emissions might increase. Flue gas recirculation (FGR) also has an adverse effect on the efficiency of the combustion process in much the same manner as excess combustion air.
Another approach for reducing the production of NO.sub.x involves changing the composition of the air/gas mixture. For example, if a mixture of oxygen and an inert gas, other than nitrogen, is utilized as the combustion atmosphere, NO.sub.x emissions are reduced. Alternatively, an additive can be introduced into the combustion chamber to form reducing agents which react with the nitrogen oxides to produce nitrogen, thus reducing the production of NO.sub.x. Thus, there are many approaches for reducing NO.sub.x emissions.
All of the foregoing approaches for reducing NO.sub.x emissions have certain inherent disadvantages with respect to cost, reliability, performance, etc. For example, reducing the combustion temperature to reduce the production of NO.sub.x may result in a reduction in the heat flux produced by the burner. Multi-stage combustion usually requires a significant amount of equipment and associated controls, all of which can be quite costly. Similarly, flue gas recirculation techniques require additional equipment and might increase the production of carbon monoxide (CO), whereas the use of additives increases operating costs. Radiant process fibrous materials are expensive, often fragile, and sensitive to blockage from airborne dust, thus requiring filtration equipment and associated maintenance. Such air filtration equipment will not prevent burner plugging problems inherent in the combustion of numerous fuels which contain contaminants, such as tar.
It is well established that thermal NO formation is the predominant NO.sub.x producing mechanism in the combustion of clean fuels, e.g., natural gas, and that the Zeldovich chain reaction mechanism applies to thermal NO formation. The chemical reaction kinetics of this analytical model predict that NO.sub.x production increases with time and temperature. These trends have been verified in practical combustion systems with peak NO.sub.x formation rates occurring slightly to the fuel lean side of stoichiometric. Reducing the combustion reaction (flame) temperature by using an excess of combustion air or FGR can, in certain cases, result in lower NO.sub.x formation. This effect can only be used to significant advantage with a homogeneous pre-mix type combustion apparatus; in chemical parlance, a plug flow reactor. In the plug flow method, the peak fuel to air concentration equals the average concentration due to the premixing. This results in the average flame temperature being equal to the peak flame temperature. The NO.sub.x emissions are then proportional to this temperature level. In a nozzle mixing burner (stirred reactor), the mixing and combustion reactions occur virtually simultaneously, and due to mixing imperfections, wide variations in fuel to air concentrations occur. This results in mixture stratification with some localized peak fuel to air concentrations significantly in excess of the overall average value. Where the higher concentrations occur, high temperatures result, with concurrent high levels of NO.sub.x formation.
Pre-mix combustion systems also offer the advantage of a high heat release rate per unit of combustion volume as compared to nozzle mix systems. In other respects, they are inferior to nozzle mixing systems; particularly with respect to combustion stability limits. Beyond certain air to fuel ratio values, combustion moves away from the burner apparatus and the flame is extinguished. These effects are illustrated in FIG. 1, in which it can be seen that pre-mix burners have a limited stability range in the more useful fuel lean non-polluting operating range. Also, for all burner types, as the stability limits are approached, the combustion efficiency decreases prior to flame extinction or "blow-out". The reduction in combustion efficiency produces large amounts of unburned combustible pollutants, predominately CO in the case of natural gas combustion.
The concept of "residence time" upon NO.sub.x formation has not attracted significant attention. Predictions of the relative contributions of time and temperature in the formation of NO using the Zeldovich chain reaction model are illustrated in FIG. 2. This Figure also illustrates the importance of "residence time" in the formation of NO.sub.x. At a flame temperature of 3400.degree. F. (2144.degree. K.), "residence times" of 0.1, 0.7 and 4.5 seconds produce NO.sub.x levels of 100 ppmv, 1000 ppmv and equilibrium levels, respectively, all of which exceed SCAQMD emissions standards (Rule 1146.1). The dependency between time and temperature in the formation of NO.sub.x is also illustrated in FIG. 3 which shows that as temperature is increased (equivalence ratio above 0.4), NO.sub.x formation is dependent upon "residence time".
In addressing the NO.sub.x problem, it is necessary that NO.sub.x and CO be considered simultaneously, because a reduction in one pollutant may merely represent a compromise with respect to emissions of the other. For most conventional burners, CO and NO.sub.x emissions are generally produced in inverse proportions. Whereas the elimination of carbonaceous pollutants, e.g., CO, etc., is amenable to relatively simple techniques, the simultaneous control of both NO.sub.x and CO has presented problems using generally accepted control techniques. This problem occurs since CO requires time and a relatively high temperature, typically of the order of 2500.degree. F. (1644.degree. K.), to oxidize such to carbon dioxide (CO.sub.2). Temperatures in excess of 2800.degree. F. (1810.degree. K.) have been found to be conducive to NO.sub.x formation. These factors can be understood by referring to FIG. 4 which is a graph of the NO.sub.x versus combustibles, such as CO, and illustrates the "emissions window" in which burners are considered to be operating within currently acceptable emission levels.
To sustain clean, efficient combustion, a region of stable burning must be created. In the absence of such, flame extinction or "blow-out" will occur. Combustion efficiency and flame stability are closely interrelated, the "blow-out" condition representing the case of zero combustion efficiency. Flame stabilization can be achieved by the use of a flame holding device or bluff body in the air/gas mixture stream. Typical flame stabilizing devices include metal screens, rods, and flame inserts. It has been found that these flame stabilizing devices also reduce NO.sub.x emissions. Radiant fiber and ceramic surface burners have also been used for similar reasons. In the foregoing cases, the rods or surfaces provide a heat absorbing mechanism capable of re-radiating the absorbed heat to an absorbing surface beyond the flame region. By such means the flame temperature is reduced with concurrent reductions in NO.sub.x formation. A key element in this approach is the ability of the radiant emitter surface to remove a substantial proportion of the heat generated, thereby controlling flame temperature. Experimental evidence of this phenomena shows an increase in NO.sub.x emissions as the heat flux to the emitter is increased. This since, for a fixed emitter geometry, i.e., surface area, the amount of heat radiation from the reaction zone is essentially constant, thereby impairing its ability to control the reaction temperature at the higher heat flux rates. Surface burners change from radiant to a blue flame mode as the heat flux (BTU/hr/ins.sup.2) is increased. In general, at heat fluxes in excess of 1000 BTU/hr/ins.sup.2, the more common surface burners "blow-out"; prior to this large quantities of CO are also produced.
In view of the foregoing, it has become desirable to develop a burner structure and/or a methodology for operating same which minimizes the production of NO.sub.x and produces low levels of CO so as to remain within the "emissions window" throughout the firing range from low to high fire.